•
Coordination time = "0.050" s (10 ms + maximal transmission time)
•
Carrier Min Dur = "0.1" s
Permissive underreaching transfer trip PUTT
In the permissive underreach scheme, a permission to trip is obtained locally from
the underreaching function and sent to the remote end. The received signal is then
combined with the local fault indication from the overreaching function resulting in
a trip command after a settable pick-up delay
Coordination time.
The protection relay of either feeder end sends a permissive signal to trip the
other end. The communication channel must be able to receive and transmit at
the same time. In permissive schemes, the communication channel must be fast
and secure, but also dependable. Inadequate security causes unwanted tripping
for external faults. Inadequate speed or dependability causes delayed tripping for
internal faults.
The
CS_PRM
signal is issued based on the underreaching residual overcurrent
protection function.
The permissive scheme is not applicable on short feeder lengths because the
residual overcurrent protection cannot easily distinguish between the internal and
external faults in such cases.
The residual overcurrent function may start only at one end due to the system
conditions such as weak zero-sequence source at one end, or high fault resistance.
However, fast tripping from both ends can be achieved by connecting a general
forward start signal to
CACC
. So, even if the local overreaching residual overcurrent
function has not picked up, the local
OPERATE
output is activated. The function also
generates a
CS_PRM
signal by combining the carrier received
CR_PRM
signal from a
remote end and also the local activation of
CACC
, giving the permission to a remote
function to trip instantaneously.
The
CR_PRM
signal must be received when the overreaching residual overcurrent
protection is still active to achieve an instantaneous trip. In some cases, due to the
fault current distribution, the overreaching protection can operate only after the
fault has been cleared by the remote terminal. In this case, there is a risk that if the
remote terminal is tripped directly by the instantaneous underreaching function,
the
CS_PRM
signal that it issues resets before the overreaching function of the local
end terminal is operated. To ensure a sufficient duration of the
CR_PRM
signal, the
CS_PRM
signal can be prolonged by time
Carrier Min Dur. The recommended value of
the
Carrier Min Dur setting is "0.1" s.
The received communication signal is combined with the output from the local
overreaching function. Therefore, there is less concern about a false signal causing
an incorrect trip. Therefore, the timer
Coordination time can be set to "0". A
communication channel failure does not affect the selectivity, but delays the
tripping at the other end for certain fault locations.
Setting guidelines for the permissive underreach scheme PUTT
•
Scheme type = "Permissive Underreach"
•
Coordination time = "0" s
•
Carrier Min Dur = "0.1" s
Permissive overreaching transfer trip POTT
In the permissive overreach scheme, the permission to trip is obtained locally from
an overreaching function and sent to the remote end. The received signal is then
Protection related functions
1MRS759142 F
1260
REX640
Technical Manual